Radiator and cooling system

ABSTRACT

A radiator includes an inlet line and an outlet line for a coolant to be cooled, and a plurality of flow passage units each being connected to the inlet and outlet lines respectively in parallel with adjacent flow passages with a space therebetween. Due to this configuration, it is possible to attain a high cooling capability with a small flow speed of the coolant, which is enabled by increasing the number of the flow passage units with respect to a certain flow rate passing through the inlet and outlet lines. This radiator can be employed for a cooling system together with a fan blowing air toward the flow passage units of the radiator, a coolant jacket for absorbing heat from a heat source such as a CPU, and a fluid pump for circulating the coolant between the coolant jacket and the radiator.

CLAIM OF PRIORITY

This application claims benefit of the Japanese Patent Application No. 2006-282682 filed on Oct. 17, 2006, and the Japanese Patent Application No. 2007-029774 filed on Feb. 8, 2007, which are hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to a radiator for cooling a fluid passing there through and a cooling system.

2. Description of the Related Art

Basic components of a cooling system, for example, a heat generating CPU (a heat source) have included a coolant jacket for absorbing heat from the CPU in contact therewith, a radiator and a fluid pump for circulating a coolant between the coolant jacket and the radiator. With regard to the individual components of such cooling systems, considerable efforts have been devoted which are intended for achieving further compactness and reliability that allow the components to be installed in a compact apparatus such as a notebook PC. These efforts are described, for example, in Japanese Unexamined Patent Application Publication No. H06-97338, U.S. Published Patent Application 2004/0042171, and U.S. Published Patent Application 2002/0195238.

Known radiators, however, are difficult to install in a low-profile compact apparatus such as a notebook PC, or are not sufficiently capable of heat radiation for such uses.

SUMMARY

The present invention provides a low-profile compact radiator which can be installed in a narrow space and has a superior capability of heat radiation, and also provides a cooling system having excellent cooling performance.

A radiator is disclosed that comprises an inlet line and an outlet line for a coolant to be cooled, and a plurality of flow passage units each having a flow passage for the coolant flowing in from the inlet line and returning to the outlet line. The flow passage is connected between the inlet line and the outlet line in parallel with adjacent flow passages with a space there between.

In one embodiment, each of the flow passage units is formed specifically, for example, by joining a pair of flow passage plates together. The flow passage plates form a fluid flow passage, which is bent into a U-shape at least once. An inlet port and an outlet port, which are respectively connected to the inlet line and the outlet line and constitute a part of the inlet line and the outlet line, respectively, can be provided in each of the flow passage plates.

The above radiator can be employed for a cooling system together with a fan blowing air toward flow passage units of this radiator, a coolant jacket for absorbing heat from a heat source in contact therewith, and a fluid pump for circulating the coolant between the coolant jacket and the radiator. The disclosed cooling system may be characterized in that the radiator is disposed so that the inlet line is further away from the fan than the outlet line, and may also be characterized in that the fan is a centrifugal fan blowing air in a centrifugal direction, and is disposed so that, when the fan blows air toward the flow passage units, the volume of air delivered from the fan toward a second end side of the flow passage units, where coolant flow passages turn from the inlet line to the outlet line, is larger than toward a first end side of the flow passage units, where the inlet and outlet lines are located.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a basic conception of a radiator according to an embodiment;

FIG. 2 is an exploded perspective view showing an embodiment of the radiator;

FIG. 3 is a plan view of the same as above;

FIG. 4 is a cross-sectional view as taken along line IV-IV of FIG. 3;

FIG. 5 is a cross-sectional view as taken along line V-V of FIG. 3;

FIG. 6 is a magnified cross-sectional view of a part of FIG. 3;

FIG. 7 is a plan view of a single flow passage plate used for the radiator shown in FIGS. 2 through 6;

FIG. 8 is a cross-sectional view of the same single flow passage plate as above;

FIG. 9 is a schematic arrangement view of a cooling system including the radiator according to an embodiment;

FIG. 10 is an external view of the cooling fan shown in FIG. 9;

FIG. 11 is a graph showing a distribution of the air volume delivered by the cooling fan; and

FIG. 12 is a schematic arrangement view of a known air cooling system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 9 is a conceptual view showing a cooling system (water cooling system) according to an embodiment of the present invention, the cooling system exemplarily having a CPU 1 as a heat source. The CPU 1 is in contact with a CPU jacket (coolant jacket) 10, which is a heat sink block. A coolant fed from a fluid pump 12 to the CPU jacket 10 absorbs heat from the CPU 1 while flowing through flow passages in the CPU jacket 10. The coolant heated in the CPU jacket 10 is cooled while flowing through coolant flow passages 21 in a radiator 20 by cooling air delivered by a cooling fan 22 and returns to the fluid pump 12; this circulation is subsequently repeated.

FIG. 1 is an explanatory view showing a basic conception of the radiator 20. An inlet line 23 and an outlet line 24 for the coolant to be cooled may be disposed in parallel to each other. A plurality of flow passage units 30 may be connected to the inlet line (inlet flow passage) 23 and the outlet line (outlet flow passage) 24 in parallel to each other with a space therebetween. That is, each of the flow passage units 30 includes an inlet port 31 connected to (coupled to) the inlet line 23, an outlet port 32 connected to (coupled to) the outlet line 24, and the coolant flow passage 21 formed between the inlet port 31 and the outlet port 32, the flow passage units 30 in a stacked state being arranged so as to be spaced apart from each other at least in the portion of the coolant flow passage 21. In an example of the cooling system shown in FIG. 9, the inlet line 23 is connected to the CPU jacket 10 and the outlet line 24 is connected to the fluid pump 12.

In the above radiator 20, if a flow rate of coolant passing through the inlet line 23 and the outlet line 24 is represented as “A”, and the number of the flow passage units 30 is represented as “n”, the flow rate of coolant passing through each of the flow passage units 30 can be expressed as “A/n”. Accordingly, it becomes possible to reduce the flow rate passing through each of the flow passage units 30 by increasing the number of the flow passage units 30 with respect to a certain flow rate passing through the inlet and outlet lines 23, 24, by which a sufficient cooling effect can be obtained.

FIGS. 2 to 8 show a more specific embodiment of the radiator 20. In the example shown in FIGS. 2 to 6, the flow passage units 30 are stacked five high; each of the flow passage units 30 has the same structure except the lowermost flow passage unit 30.

Each of the flow passage units 30 is composed of a pair of flow passage plates 34U, 34L, which are joined together in face to face relationship. The flow passage plates 34U, 34L are each formed, for example, of a thermally conductive metal material by pressing, and each have a symmetrical shape (identical shape) with respect to the joint face (face where the flow passage plates are joined to each other). FIGS. 7 and 8 show the shape of a flow passage plate 34U (34L), which is formed in a slender shape and has a recessed flow passage portion 36 in its flat joint face 35, the recessed flow passage portion 36 being formed in a flat U-shape. The inlet port 31 and the outlet port 32 are provided at the end portion of U-shape legs of the U-shaped recessed flow passage portion 36 (end portion opposite the U-turn portion), the inlet port 31 and the outlet port 32 each being formed by making a hole.

The inlet port 31 and the outlet port 32 are respectively formed in spacer portions 37, 38, which are formed so as to project outwardly from the U-shaped recessed flow passage portion 36. A spacer protrusion 39, having the same height as the spacer portion 37 (38), is provided in the end portion opposite the spacer portion 37 (38) on the flow passage plate 34U (34L).

The above flow passage plates 34U, 34L are put together in opposite directions so that their recessed flow passage portions 36 each face outward, and are joined together at their joint faces 35, for example, by brazing. As a result, the coolant flow passage 21 is formed with the U-shaped recessed flow passage portions 36 protruding in opposite directions. As can be seen in FIG. 5, the coolant flow passage 21 has a low-profile. When the spacer portions 37 (38) of a couple of upper and lower flow passage units 30 are joined together, the inlet ports 31 of the respective flow passage units 30 and the outlet ports 32 of the respective flow passage units 30 are connected each other; whereby the inlet line 23 and the outlet line 24 are each partially formed. The inlet port 31 (outlet port 32) is not provided in the spacer portion 37 (38) of the lower flow passage plate 34L of the lowermost flow passage unit 30 (see FIG. 6). The lowermost flow passage unit 30 can be obtained by not making the holes used as the inlet and outlet ports 31, 32 respectively, which are illustrated by a two-dot chain line in FIG. 7. The spacer protrusions 39 of a couple of upper and lower flow passage units 30 are joined together simultaneously when the spacer portions 37 (38) of a couple of upper and lower flow passage units 30 are joined together, and allow a cooling air passage S to be configured between the U-shaped recessed flow passage portions 36 (coolant flow passages 21) of the respective upper and lower flow passage units 30.

The five stacked flow passage units 30 stacked as described above are joined together by a flow passage block 40, which includes an upper body 41 having the inlet line 23 and the outlet line 24, and a lower body 42; the upper and lower bodies 41, 42 fix the five-stacked flow passage units 30 by holding them therebetween. On the upper body 41, there are formed a pair of annular-shaped recessed portions 43 into which the spacer portions 37 (38) of the uppermost flow passage unit 30 are each inserted using an O-ring 44, by which sealing of the coolant can be achieved (see FIG. 6). A tightening distance between the upper and lower bodies 41, 42 is limited by a pair of spacer legs 41S.

In the above radiator 20, accordingly, the coolant to be cooled is supplied from the inlet line 23 and is distributed to the individual passage units 30. That is, the coolant is fed from the inlet port 31 of each of the flow passage units 30 to the coolant flow passage 21, and is returned through the outlet port 32 to the outlet line. Since there is formed a cooling air passage S between a couple of upper and lower flow passage units 30, the air passing through the passage can cool the coolant flowing in the coolant flow passages 21 through heat exchange with the flow passage plates 34U (34L). Due to the configuration that the flow passage units 30 are disposed in parallel, the total cross-sectional flow passage area of the whole flow passage units 30 can be enlarged, which allows the flow speed of the coolant to be reduced so that a sufficient cooling effect can be obtained. Moreover, the cooling performance can be freely specified by selecting the number of the flow passage units 30 to be stacked.

Although a U-shaped flow passage is configured for the flow passage unit 30 (flow passage plate 34U (34L)) in this embodiment, it is possible to form such a flow passage as an S-shaped one or one having a plurality of turns. An advantage of this embodiment is that the spacer protrusion 39 can keep the distance between the flow passage units 30 in balance by being disposed on the opposite side of the spacer portion 37 (38) with respect to the longitudinal direction. However, such a spacer protrusion can be disposed at a different position. It is also possible not to provide the spacer protrusion 39 if the distance between the flow passage units 30 can be kept in balance.

Next, a cooling system according to the disclosed embodiment will be described with reference to FIGS. 9 to 11.

This cooling system L is a water cooling system that is constituted by the above radiator 20, a cooling fan 22 for blowing air toward coolant flow passages 21 of the radiator 20, a CPU jacket (coolant jacket) 10 being in contact with a CPU 1 and absorbing heat therefrom, and a fluid pump 12 for circulating a coolant between the CPU jacket 10 and the radiator 20. An inlet line 23 for the coolant to be cooled is located on the near side with respect to the cooling fan 22, and an outlet line 24 is located on the far side.

The cooling fan 22 may be a multi-blade centrifugal fan having an air-blowing port 22 a located in a portion arranged in a centrifugal direction, and may be disposed so that the air-blowing port 22 a faces the radiator 20, and also so as to blow air toward a second end side 20 b where coolant flow passages turn from the inlet line 23 to the outlet line 24 more than toward a first end side 20 a where the inlet and outlet lines 23, 24 are located. FIG. 10 is an external view showing the cooling fan 22, and FIG. 11 shows a distribution of the air volume delivered by the cooling fan 22. This distribution was obtained through a measurement such that the velocities of airflow from the cooling fan 22 positioned with its face upward were measured by wind speed sensors S1 to S7 located at different points in proximity to the air-blowing port 22 a (see FIG. 10). The rotational direction of the cooling fan 22 was counterclockwise when the measurement was carried out. The positions of the wind speed sensors S1 to S7 are indicated with the distance from the lowermost end, which is defined as zero, of the air-blowing port 22 a shown in the attached drawing (lower side of FIG. 10). Since the air delivered from the cooling fan 22 flows in a direction toward the upper-right from the lower-left in FIG. 10 with a large angle of several tens of degrees when the cooling fan 22 is positioned with its face upward, the wind speed is large in the upper portion of the air-blowing port 22 a (upper side in FIG. 10) and is small in the lower portion of the air-blowing port 22 a (lower side in FIG. 10) as can be seen in FIG. 11. In the case of a cooling fan having an air volume distribution that varies according to positions of its air-blowing port 22 a, as in this case, the air volume distribution is reversed when the cooling fan is turned upside down. That is, when the cooling fan 22 is positioned with its back face upward, the air delivered from the cooling fan 22 flows in the direction toward the lower-right from the upper-left in FIG. 10 with a large angle, so the wind speed is large in the lower portion of the air-blowing port 22 a and is small in the upper portion of the air-blowing port 22 a in the attached drawing. In this cooling system L, taking another reason described later into consideration, the cooling fan 22 is laid with its face upward so that the wind speed is small in the lower portion of the air-blowing port 22 a facing the first end side 20 a of the radiator 20 (the side where the inlet and outlet lines 23, 24 are located), and is large in the upper portion of the air-blowing port 22 a facing the second end side 20 b of the radiator 20 (the side where coolant flow passages turn).

Now, the cooling performance of this cooling system L (shown in FIG. 9) will be evaluated.

FIG. 12 shows a configuration of an air cooling system A as an example to be compared with this cooling system L. The air cooling system A is constituted by a heat pipe 111 for absorbing heat from a CPU 1 via a CPU jacket 10, a radiator 120 into which a heat dissipating portion 111 b of the heat pipe 111 (the end portion opposite a heat absorbing portion 111 a in contact with the CPU 1) is inserted, and a cooling fan 122 for blowing air toward the radiator 120. In the heat pipe 111, there is enclosed a heat medium, which evaporates in the heat absorbing portion 111 a upon reception of heat from the CPU 1, carries heat by travelling in a vapor state to the heat dissipating portion 111 b, and returns to the heat absorbing portion 111 a in a condensed state after being cooled in the heat dissipating portion 111 b by the radiator 20; the CPU 1 can be cooled by this circulation being subsequently repeated. The CPU 1 serving as a heat source and the cooling fan 122 used for this air cooling system A are the same as ones as used for the cooling system L.

A comparison in cooling performance between the cooling system L (the present embodiment) and the air cooling system A (a comparable example) is shown in Table 1.

TABLE 1 Cooling performance Present Comparable embodiment example (Liquid (Air Cooling) Cooling) a01; Heater Power (CPU) W 42.8 42.8 a02; Fan size (Air) mm × mm 12.5 × 45 12.5 × 45 a03; Fan wind speed (DC 5 V) m/s 4.0 4.0 a04; Fan wind speed m/s 2.8 1.5 (Radiator out) a05; Ambient Ta ° C. 25.4 25.4 a06; Radiator out air T ° C. 41.0 74.5 a07; Heater (CPU) T ° C. 72.8 99.6 a08; Jacket in T ° C. 59.9 — a09; Jacket out T ° C. 65.6 93.1 a10; Radiator in T ° C. 65.6 83.1 a11; Radiator out T ° C. 60.7 — a12; Jacket

T ° C.

5.7

6.5 a13; Radiator-Jacket/Heat Pipe

T ° C.

0.8

10.0 a14; Radiator Liquid

T ° C.

4.9 — a15; Radiator Air

T ° C.

15.6

49.1 a16; Thermal resistance Jacket ° C./W 0.17 (0.15) a17; Thermal resistance H-Pipe ° C./W — (0.23) a18; Thermal resistance Radiator ° C./W 0.94 (1.35) a19; Thermal resistance Over all ° C./W 1.11 1.73

In Table 1, the first item “a1” indicates the power [W] of the heat source (CPU 1), the second item “a2” indicates the size of the cooling fan represented by internal dimensions [mm×mm] of its air-blowing port, the third item “a3” indicates the wind speed [m/s] at the air-blowing port of the cooling fan (power supply of DC5V), the fourth item “a4” indicates the wind speed [m/s] at a discharge end of the radiator, the fifth item “a5” indicates the ambient temperature [Ta° C.], the sixth item “a6” indicates the air temperature [T° C.] of the wind discharged from the radiator, and the seventh item “a7” indicates the temperature [T° C.] of the heat source (CPU 1). The eighth item “a8” and the ninth item “a9” for the cooling system L, respectively, indicate the coolant temperatures at the inlet side and the outlet side of the CPU jacket 10, and the ninth item “a9” for the air cooling system A indicates the temperature of the heat absorbing portion 111 a of the heat pipe 111 (measuring point “a9” in FIG. 12), assuming that it represents the temperature at an outlet side of the CPU jacket 10. The tenth item “a10” and the eleventh item “a11” for the cooling system L, respectively, indicate the coolant temperature at the inlet side and the outlet side of the radiator 20, and the tenth item “a10” for the air cooling system A indicates the temperature that is measured in proximity to the heat dissipating portion 111 b of the heat pipe 111 (measuring point “a10” in FIG. 12) inserted into the radiator 120, assuming that it represents the temperature at an inlet side of the radiator 120. The twelfth item “a12” indicates the coolant temperature difference [ΔT° C.] between the inlet side and the outlet side of the CPU jacket 10 for the cooling system L, and also indicates the temperature difference [ΔT° C.] between the CPU 1 and the heat absorbing portion 111 a of the heat pipe 111. The thirteenth item “a13” indicates the coolant temperature difference [ΔT° C.] between the outlet side of the radiator 20 and the inlet side of the CPU jacket 10 for the cooling system L, and also indicates the temperature difference between the inlet side of the radiator 120 and the outlet side of the CPU jacket 10, i.e., the temperature difference [ΔT° C.] between the heat absorbing portion 111 a and the temperature that is measured in proximity to the heat dissipating portion 111 b of the heat pipe 111. The fourteenth item “a14” indicates the coolant temperature difference [ΔT° C.] between the inlet side and the outlet side of the radiator 20 for the cooling system L, and the fifteenth item “a15” indicates the temperature difference [ΔT° C.] between the air discharged from the radiator 20 or 120 and the ambient air. The sixteenth item “a16” indicates the thermal resistance [° C./W] of the CPU jacket 10, the seventeenth item “a17” indicates the thermal resistance [° C./W] of the heat pipe 111, the eighteenth item “a18” indicates the thermal resistance [° C./W] of the radiators 20, 120, and the nineteenth item “a19” indicates the thermal resistance [° C./W] of the whole systems. A cooling performance can be evaluated with respect to the magnitude of the thermal resistance of the whole system, and is considered that a smaller thermal resistance allows a higher cooling performance. The thermal resistance of a whole system can be calculated by (heat source temperature “a7”−ambient temperature “a5”)/heat source power “a1”.

As can be seen in Table 1, the thermal resistance 1.11 [° C./W] of this cooling system L is smaller than the thermal resistance 1.73 [° C./W] of the air cooling system A, which means that this cooling system L provides a better cooling performance than the air cooling system A.

Next, Another comparison in cooling performance between the cooling system L (the present embodiment) and a second cooling system (second comparable example) is shown in Table 2. In the second cooling system, an outlet line 24 for a coolant is placed in a side far from a cooling fan 22 and an inlet line 23 for the coolant is placed in a side near to the cooling fan 22, while the outlet line 24 is placed in the side near to the cooling fan 22 and the inlet line 23 is placed in the side far from the cooling fan 22 in the cooling system L. The configuration of the second cooling system is identical to that of the cooling system L shown in FIG. 9 except the placement of the inlet line 23 and the outlet line 24. Description of the items “a1” to “a16”, “a18” and “a19” in Table 2 is omitted since they are identical to the items “a1” to “a16”, “a18” and “a19” in Table 1.

TABLE 2 Liquid Cooling performance Radiator

Inlet line of the radiator

Present embodiment Comparable example a01; Heater Power (CPU) W (Far from the fan) (Near to the fan) a02; Fan size (Air) mm × mm 8.5 × 57 8.5 × 57 a03; Fan wind speed (DC 5 V) m/s 6.1 6.1 a04; Fan wind speed m/s 3.4 3.4 (Radiator out) a05; Ambient Ta ° C. 25.0 25.0 a06; Radiator out air T ° C. 34.0 34.2 a07; Heater (CPU) T ° C. 64.1 65.0 a08; Jacket in T ° C. 53.3 54.0 a09; Jacket out T ° C. 58.3 59.1 a10; Radiator in T ° C. 58.3 59.1 a11; Radiator out T ° C. 54.0 54.8 a12; Jacket

T ° C.

5.0

5.1 a13; Radiator-Jacket/Heat Pipe

T ° C.

0.7

0.8 a14; Radiator Liquid

T ° C.

4.3

4.3 a15; Radiator Air

T ° C.

9.0

9.2 a16; Thermal resistance Jacket ° C./W 0.14 0.15 a18; Thermal resistance Radiator ° C./W 0.82 0.84 a19; Thermal resistance Over all ° C./W 0.97 0.99

As can be seen in Table 2, the thermal resistance 0.97 [° C./W] of the cooling system L (the present embodiment) is smaller than the thermal resistance 0.99 [° C./W] of the second cooling system (the comparable example), i.e., the cooling performance of the cooling system L, in which the inlet line 23 for the coolant is placed in the side far from the cooling fan 22 and the outlet line 24 is placed in the side near to the cooling fan 22, is enhanced from that of the second cooling system, in which the outlet line 24 for the coolant is placed in the side far from the cooling fan 22 and the inlet line 23 is placed in the side near to the cooling fan 22.

The reason why the difference in cooling performance arises from the different arrangements of the inlet line 23 and the outlet line 24 is thought to be due to the fact that the cooling system L (the present embodiment) allows the coolant to be effectively cooled by the lowest-temperature cooling air in the side of the outlet line 24, whereas the second cooling system (the comparable example) reduces its cooling efficiency due to the fact that the coolant is subject to hot air, which has been heated by the hot coolant passing flow passages in the side of the inlet line 23, when passing the flow passages in the side of the outlet line 24 after the coolant has been once cooled.

In the next place, a further comparison in cooling performance between the cooling system L (the present embodiment) and a third cooling system (third comparable example) is shown in Table 3. In the third cooling system, a cooling fan 22 is disposed so as to blow air toward a first end side 20 a (the side where inlet and outlet lines 23, 24 are located) more than toward a second end side 20 b (the side where coolant flow passages turn), while the cooling fan 22 is disposed so as to blow air toward the second end side 20 b more than toward the first end side 20 a in the cooling system L. The configuration of the third cooling system is identical to that of the cooling system L shown in FIG. 9 except the orientation of the cooling fan 22. Description of the items “a1” to “a16”, “a18” and “a19” in Table 3 is omitted since they are identical to the items “a1” to “a16”, “a18” and “a19” in Table 1.

TABLE 3 Liquid Cooling performance Orientation of the fan Present embodiment Comparable example (Its face upward) (Its back upward) a01; Heater Power (CPU) W 40.4 40.4 a02; Fan size (Air) mm × mm 8.5 × 57 8.5 × 57 a03; Fan wind speed (DC 5 V) m/s 6.1 6.1 a04; Fan wind speed m/s 3.4 3.4 (Radiator out) a05; Ambient Ta ° C. 25.0 25.0 a06; Radiator out air T ° C. 34.0 35.4 a07; Heater (CPU) T ° C. 64.1 65.9 a08; Jacket in T ° C. 53.3 55.1 a09; Jacket out T ° C. 58.3 60.1 a10; Radiator in T ° C. 58.3 60.1 a11; Radiator out T ° C. 54.0 55.8 a12; Jacket

T ° C.

5.0

5.0 a13; Radiator-Jacket/Heat Pipe

T ° C.

0.7

0.7 a14; Radiator Liquid

T ° C.

4.3

4.3 a15; Radiator Air

T ° C.

9.0

10.4 a16; Thermal resistance Jacket ° C./W 0.14 0.14 a18; Thermal resistance Radiator ° C./W 0.82 0.87 a19; Thermal resistance Over all ° C./W 0.97 1.01

As can be seen in Table 3, the thermal resistance 0.97 [° C./W] of the cooling system L (the present embodiment) is smaller than the thermal resistance 1.01 [° C./W] of the third cooling system (the comparable example), i.e., the cooling performance of the cooling system L, in which the cooling fan 22 blows air toward the second end side 20 b more than toward the first end side 20 a, is enhanced from that of the third cooling system, in which the cooling fan 22 blows air toward the first end side 20 a more than toward the second end side 20 b.

The reason why the difference in cooling performance arises from the different orientation of the fan is thought to be due to the fact that the airflow passage in the first end side 20 a, in which the inlet and outlet lines 23, 24 are located, is smaller than that in the second end side 20 b, and is also thought to be the fact that the air delivered from the cooling fan 22 flows not in the direction perpendicular to the air-blowing port 22 a but in the direction greatly angled with respect to the air-blowing port 22 a as explained in FIGS. 10 and 11. That is, in the case of the third cooling system (the comparable example), the air delivered from the cooling fan 22 in FIG. 9 flows from the left side of the second end side 20 b of the radiator 20 toward the right side of the first end side 20 a, which means that the airflow generated by the cooling fan 22 is blocked by a flow passage block 40 and subject to further pressure loss; as a result, the volume of cooling air actually delivered to the radiator 20 is reduced and the effect of cooling the radiator 20 is reduced. On the other hand, in this cooling system L (the present embodiment), the air delivered from the cooling fan 22 in FIG. 9 flows from the left side of the first end side 20 a of the radiator 20 toward the right side of the second end side 20 b, where the airflow generated by the cooling fan 22 is not blocked so much as it is directed to the right side of the first end side 20 a and passes smoothly through the radiator 20 and subject to less pressure loss; as a result, the volume of cooling air actually delivered to the radiator 20 becomes large. This is considered to be a reason why the effect of cooling the radiator 20 in this cooling system L is increased compared to the third cooling system. Since a cooling effect greatly depends on a difference in temperatures between cooling air and an object for cooling, the following is considered to be another reason. In this cooling system L, cooling air having a lowest temperature close to ambient temperature strikes an area (the side of outlet line 24) where the coolant temperature is lowest and the difference with the cooling air temperature is large, so the coolant temperature in the outlet is further decreased and a higher cooling effect is obtained. On the other hand, in the third cooling system (the comparable example), cooling air, which has been once heated in the first end side 20 a, strikes an area (the side of outlet line 24) where the coolant temperature has been most lowered, which is supposed to be a reason why the coolant temperature is not decreased at the portion around the outlet of the coolant. 

1. A radiator comprising: an inlet line and an outlet line for a coolant to be cooled; and a plurality of flow passage units each having a fluid flow passage for the coolant flowing in from the inlet line and returning to the outlet line, the fluid flow passage being connected between the inlet line and the outlet line in parallel with adjacent fluid flow passages with a space therebetween.
 2. The radiator according to claim 1, wherein each of the flow passage units comprises a pair of flow passage plates joined together, the flow passage plates forms a fluid flow passage, which is bent into a U-shape at least once, and each being provided with an inlet port and an outlet port, which are respectively connected to the inlet line and the outlet line, and constitute a part of the inlet line and the outlet line, respectively.
 3. The radiator according to claim 2, wherein the pair of flow passage plates constituting each of the flow passage units are symmetric to each other with respect to the face where the flow passage plates are joined, and each have a recessed flow passage portion formed in a flat U-shape, and each have an inlet port and an outlet port formed in one end portion and the other end portion of the recessed flow passage portion, respectively.
 4. The radiator according to claim 2, wherein the pair of flow passage plates constituting each of the flow passage units each have spacer portions formed so as to project outwardly from the inlet and outlet portions respectively, the spacer portions allowing an airflow passage in a remaining space between flow passage units to be provided when in contact with the spacer portions of the adjacent flow passage unit.
 5. The radiator according to claim 2, wherein the inlet ports and the outlet ports of the plurality of stacked flow passage units are respectively connected each other, and further connected to the inlet line and the outlet line, respectively.
 6. The radiator according to claim 2, wherein the pair of flow passage plates constituting each of the flow passage units each have a spacer protrusion integrated with the flow passage plate for securing a space between flow passage units when in contact with the spacer protrusion of the adjacent flow passage unit.
 7. A cooling system comprising: a radiator comprising: an inlet line and an outlet line for a coolant to be cooled; and a plurality of flow passage units each having a fluid flow passage for the coolant flowing in from the inlet line and returning to the outlet line, the fluid flow passage being connected between the inlet line and the outlet line in parallel with adjacent fluid flow passages with a space therebetween; a fan that blows air toward flow passage units of the radiator; a coolant jacket that absorbs heat from a heat source in contact therewith; and a fluid pump that circulates a coolant between the coolant jacket and the radiator, wherein the radiator is disposed so that its inlet line is further away from the fan than its outlet line.
 8. A cooling system comprising: a radiator comprising: an inlet line and an outlet line for a coolant to be cooled; and a plurality of flow passage units each having a fluid flow passage for the coolant flowing in from the inlet line and returning to the outlet line, the fluid flow passage being connected between the inlet line and the outlet line in parallel with adjacent fluid flow passages with a space therebetween; a fan that that blows air toward flow passage units of the radiator; a coolant jacket that absorbs heat from a heat source in contact therewith; and a fluid pump that circulates a coolant between the coolant jacket and the radiator, wherein the fan is a centrifugal fan blowing air in a centrifugal direction, and is disposed so that, when the fan blows air toward the flow passage units, the volume of air delivered by the fan toward a second end side of the flow passage units, where coolant flow passages turn from the inlet line to the outlet line of the flow passage units, is larger than that delivered toward a first end side of the flow passage units, where the inlet and outlet lines are located. 